1. Field of the Invention
The present invention relates to an automatic gain control (AGC) amplifier for compensating a cable loss, more particularly, to an AGC amplifier for compensating a cable loss, having a wide compensation ability and adaptable to the forming of a large scale integration (LSI) circuit construction.
2. Description of the Related Art
An equalizing amplifier system, which carries out a wave shaping of a pulse signal having an amplitude which is attenuated by a cable, for example, a coaxial cable or a paired-cable, to a pulse signal having a predetermined equalization characteristic, usually includes a build-out network (BON), a preamplifier, an AGC amplifier, a wave equalizer, a post-amplifier, a peak detector connected to the post-amplifier to detect a peak of an output from the post-amplifier, and a DC amplifier connected to the peak detector and supplying an amplified peak signal to the AGC amplifier.
The BON is an adjustment circuit enabling a level compensation at the AGC amplifier even when a cable relaying distance is short and thus an input level is too high. The preamplifier is a low noise amplifier for suppressing noise. The AGC amplifier compensates cable loss to restore an amplitude so that a gain thereof is controlled by the peak detector and the DC amplifier to restore a peak of a wave equalized at the equalizer. The equalizer equalizes the AGC amplified signal to form a signal having a predetermined wave characteristic, in accordance with a predetermined equalization function. The post-amplifier supplies a high level signal to a timing circuit and a discrimination and recovery circuit.
The present invention pertains to the AGC amplifier. A Bode type variable equalizer and modifications thereof are used as an AGC amplifier in a frequency division multiplexing (FDM) system, but a variable equalization range thereof is narrow, and accordingly, this type is not used for a pulse coded modulation (PCM) system. The present invention particularly relates to an AGC amplifier for compensating a cable loss in the PCM system.
Prior art AGC amplifiers, which will be described later, suffer from a narrow variable equalization range and a relatively bulky circuit size, and cannot be adapted to form an LSI circuit construction.